Supercritical Carbon Dioxide Emulsified Acid

ABSTRACT

One aspect of an emulsion includes an internal phase including acid, an external phase including supercritical carbon dioxide, and multiple nanoparticles to stabilize the internal phase and the external phase. The acid can include hydrochloric acid. The hydrochloric acid can include 15% hydrochloric acid. The nanoparticles can include hydrophobic nanoparticles. A concentration of nanoparticles in the emulsion can be at least 0.1% by weight. The emulsion can include a corrosion inhibitor. A concentration of the corrosion inhibitor can be in a range of between 0.25% and 0.6% by volume. A ratio of a concentration of the acid to a concentration of the supercritical carbon dioxide can be in a range between 30% and 70%.

TECHNICAL FIELD

This disclosure relates to stimulation of subterranean formations withan acid emulsion.

BACKGROUND

During some well stimulation procedures such as fracturing, an acid isintroduced into the wellbore. In order to achieve deep acid penetration,an acid emulsion can be used to retard the reaction rate between theacid and the formation. For example, in an acid-in-diesel emulsion, theretardation of the reaction is due to the diesel external phase whichacts as a barrier minimizing the acid transfer to the rock surface.However, at elevated temperatures (i.e., 300° F. and above), theacid-in-diesel emulsion becomes unstable, and thus the retardationmechanism is lost.

SUMMARY

This disclosure describes a supercritical carbon dioxide emulsifiedacid.

In some aspects, an emulsion includes an internal phase including acid,an external phase including supercritical carbon dioxide, and multiplenanoparticles to stabilize the internal phase and the external phase.

This, and other aspects, can include one or more of the followingfeatures. The acid can include hydrochloric acid. The hydrochloric acidcan include 15% hydrochloric acid. The nanoparticles can includehydrophobic nanoparticles. A concentration of nanoparticles in theemulsion can be at least 0.1% by weight. The emulsion can include acorrosion inhibitor. A concentration of the corrosion inhibitor can bein a range of between 0.25% and 0.6% by volume. A ratio of aconcentration of the acid to a concentration of the supercritical carbondioxide can be in a range between 30% and 70%.

In some aspects, a method of manufacturing an emulsion includes mixing afirst quantity of nanoparticles and a second quantity of carbon dioxide.The method also includes mixing a third quantity of acid with themixture of the first quantity and the second quantity at a temperatureand a pressure sufficient to convert the carbon dioxide intosupercritical carbon dioxide.

This, and other aspects, can include one or more of the followingfeatures. The method can include mixing a fourth quantity of corrosioninhibitors with the first quantity, the second quantity, and the thirdquantity. Mixing the third quantity of the acid to the mixture of thefirst quantity and the second quantity can include mixing the thirdquantity at a drop-wise rate. Mixing the first quantity, the secondquantity, and the third quantity can include mixing the first quantity,the second quantity, and the third quantity for a duration between aboutten minutes and about fifteen minutes. The temperature can be at least40 C and the pressure can be at least 1100 psi. The method can includemeasuring an emulsion height in response to mixing the third quantitywith the mixture of the first quantity and the second quantity anddetermining a stability of the emulsion based, in part, on the measuredemulsion height. Determining the stability of the emulsion based, inpart, on the measured emulsion height can include comparing the measuredemulsion height with a total emulsion height. The method can includepressurizing the second quantity of carbon dioxide before mixing thefirst quantity and the second quantity. Pressurizing the second quantityof carbon dioxide can include pressurizing the second quantity of carbondioxide to at least 1500 psi. The acid can include hydrochloric acid andthe nanoparticles can include hydrophobic nanoparticles.

In some aspects, a method for controlling formation stimulation includesidentifying a rate of acid retardation in a formation, determining aquantity of supercritical carbon dioxide to be included in a stimulantto obtain the identified rate of acid retardation in the formation; andmanufacturing an emulsion. The emulsion includes an internal phaseincluding a first quantity of acid, an external phase including thedetermined quantity of supercritical carbon dioxide, and a secondquantity of multiple nanoparticles to stabilize the internal phase andthe external phase.

This, and other aspects, can include the following feature. The methodcan include flowing the emulsion into the formation.

The details of one or more implementations of the subject matterdescribed in this disclosure are set forth in the accompanying drawingsand the description below. Other features, aspects, and advantages ofthe subject matter will become apparent from the description, thedrawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exampleacid-in-supercritical carbon dioxide emulsion.

FIG. 2 is a flow chart showing an example process 200 for producingacid-in-supercritical carbon dioxide emulsion.

FIG. 3 is a schematic diagram of an example system to manufacture anacid-in-supercritical carbon dioxide emulsion.

FIG. 4 is a diagram illustrating an example well system.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

This disclosure describes a supercritical carbon dioxide emulsifiedacid. For example, the supercritical carbon dioxide emulsified acid canbe used a treatment fluid in a wellbore.

Hydrochloric acid (HCl) has been widely used to stimulate carbonateformation due to its low cost and high dissolving power. However, HClhas high corrosion rate and acid-rock reaction rate. High corrosioninhibitor concentrations can be used particularly at high temperaturesto minimize the corrosion of downhole equipment which significantlyincrease the cost of acid treatments. Organic acids such as citric acid,formic acid, and acetic acid have low corrosivity and low acid-rockreaction rate compared to HCl. However, they are expensive and prone tocalcium- and magnesium-based precipitation. Furthermore, in comparisonto HCl, they have less dissolving power and do not react to completionunder reservoir conditions. To retard the reaction rate between HCl andcarbonate rock and achieve deep acid penetration, the HCl can beprepared as an emulsified acid, such as an acid-in-diesel emulsion. Theacid-rock reaction retardation is due to the diesel external layer whichacts as a barrier that provides corrosion protection and also minimizesthe acid transfer to the rock surface. However, efficient formationpenetration by acid-in-diesel emulsions is limited by the temperature ofthe downhole environment. At elevated temperatures (i.e., above 300°F.), the emulsion becomes unstable, and the external phase diesel nolonger acts as an effective barrier, and thus the retardation mechanismis lost.

This disclosure describes the manufacture and use of an emulsion ofacid-in-supercritical carbon dioxide (“supercritical CO₂” or “scCO₂”),including an internal phase of acid dispersed within an external phaseof supercritical CO₂. Nanoparticles are used to stabilize theacid-in-supercritical CO₂ emulsion and enable the acid-in-supercriticalCO₂ dioxide emulsion to withstand high temperatures. For example, attemperatures greater than 300° F., an acid-in-supercritical CO₂ emulsionis a more stable emulsion than an acid-in-diesel emulsion.

The acid-in-supercritical CO₂ emulsion described can be used to controlstimulation of a subterranean formation in a wellbore, for example,during a fracturing operation. The emulsion can be flowed into theformation, for example, as a treatment fluid or stimulation fluid. Asthe external phase, the supercritical CO₂ acts as a barrier thatdecreases acid contact with downhole surfaces. The acid retardation isdue to the supercritical CO₂ external layer in addition to the presenceof gaseous CO₂ as a reaction product. Thus, the supercritical CO₂provides significant corrosion protection from the acid. Furthermore,the supercritical CO₂ retards acid transfer to the surface of theformation of interest, allowing the acid to penetrate deeper into theformation. Thus, the stimulation is effective over a larger volume ofthe formation, potentially increasing extraction efficiency. As theacid-in-supercritical CO₂ emulsion has higher temperature tolerance, itcan be used reliably in downhole environment conditions, includingelevated temperature environments. The acid-in-supercritical CO₂emulsion also enables rapid and effective clean up and liquid recoveryafter wellbore stimulation. For example, supercritical CO₂ in theemulsion can transition to a gaseous phase and be vented from thewellbore as gaseous CO₂. The CO₂ can also assist in lifting spent acidout of the wellbore.

FIG. 1 is a schematic diagram illustrating an exampleacid-in-supercritical carbon dioxide (“acid-in-scCO₂”) emulsion 100. Theexample acid-in-scCO₂ emulsion includes an external phase ofsupercritical carbon dioxide 102. The supercritical CO₂ 102 is CO₂ at asufficient temperature and a sufficient pressure to reach asupercritical state. For example, CO₂ simultaneously at temperaturesgreater than 40 C and at pressures greater than 1100 psi exists in asupercritical state. Droplets of acid 104 are the internal phase of theemulsion that are dispersed in the supercritical carbon dioxide 102. Theouter surface of each droplet of acid 104 is surrounded by multiplenanoparticles 106. The nanoparticles 106 stabilize the emulsified acid104 within the supercritical carbon dioxide 102 and allow the acid 104to remain longer in the emulsified state. The acid 104 can behydrochloric acid (HCl) or another acid such as citric acid, formicacid, acetic acid, hydrofluoric acid, or another acid. In someimplementations, the acid 104 is a chelating agent (e.g., EDTA oranother chelating agent) or another type of reactive fluid. In someimplementations, one or more corrosion inhibitors are added to the acid104 or to the emulsion 100. The corrosion inhibitors can help protecttubing and equipment from corrosion due to the acid 104. For example, acorrosion inhibitor can include acetaldehyde, methanol, surfactants, orother corrosion-inhibiting substances. The nanoparticles 106 can besilicon dioxide nanoparticles or any other suitable hydrophobicnanoparticles.

FIG. 2 is a flow chart showing an example process 200 for producingacid-in-scCO₂ emulsion. In some implementations, the process 200 mayinclude additional or different operations, and the operations may beperformed in the order shown in FIG. 2 or in another order. The process200 can be repeated or the absolute amounts of materials increased ordecreased to produce more or less product, respectively.

At 210, a first quantity of nanoparticles is transferred to ahigh-pressure and high-temperature cell. The cell can be a see-throughcell, tank, chamber, or other volume capable of withstanding thetemperature and pressure required to maintain CO₂ in a supercriticalstate. The nanoparticles can be hydrophobic nanoparticles as describedpreviously. As an example implementation, 1 g of nanoparticles can betransferred to the cell. In other implementations, other amounts orconcentrations of nanoparticles can be transferred to the cell as thefirst quantity. For example, the nanoparticles can be transferred at aconcentration of 0.1% by weight to 10% by weight.

At 220, a second quantity of carbon dioxide is transferred to the celland mixed with the nanoparticles. The CO₂ can be added in a solid state,a gaseous state, a liquid state, or a supercritical state. For example,the CO₂ can be pressurized above 1500 psi and thus be in a liquid statewhen transferred to the cell. For the example implementation, 30 ml ofliquid CO₂ can be transferred to the cell and mixed with the 1 g ofnanoparticles. In other implementations, other amounts of CO₂ can betransferred to the cell as the second quantity.

At 230, sufficient heat and pressure is applied to the CO₂-nanoparticlemixture to convert the CO₂ to supercritical CO₂. In someimplementations, the sufficient temperature and pressure is achieved byheating the cell, which can increase the pressure of the CO₂ as the CO₂is heated within the cell. In some implementations, the CO₂ is in asupercritical state when added to the cell, and the cell has asufficient temperature and pressure to maintain the CO₂ in thesupercritical state. In some implementations, the cell is at asufficient temperature and pressure when CO₂ that is not in asupercritical state is transferred to the cell, and the CO₂ is convertedto the supercritical state in the cell during or after being transferredto the cell. For example, the CO₂ can be pressurized when transferred tothe cell, and the cell provides heat sufficient to convert the CO₂ to asupercritical state.

At 240, a third quantity of acid is added to the scCO₂-nanoparticlemixture in the cell while the cell maintains the CO₂ in thesupercritical state. The acid can be HCl or another acid as describedpreviously, or the acid can be a combination of acids. The acid can alsobe a diluted acid. The acid can be added at a certain rate to facilitatemixing, such as a drop-wise rate or other rate. In some implementations,the proportion of the concentration of the acid to the concentration ofthe supercritical CO₂ is in a range between 30% and 70%. In the exampleimplementation, 70 ml of 15% HCl can be added to the 30 mL of CO₂ andthe 1 g of nanoparticles. In other implementations, other amounts ofacid can be transferred to the cell as the third quantity.

In some implementations, one or more corrosion inhibitors are mixed withthe acid prior to the acid being added to the cell. In someimplementations, the corrosion inhibitors are added to mixture of acid,scCO₂, and nanoparticles. In some implementations, the concentration ofthe corrosion inhibitor is in a range of between 0.25% and 0.6% byvolume.

At 250, the acid-scCO₂-nanoparticle mixture is blended for a duration oftime to obtain an acid-in-scCO₂ emulsion. The mixture can be blended inthe cell and can be blended using a high shear mixer, a stirrer, anagitator, or another blending device. The mixture can be blended untilan emulsion is formed. For example, in the example implementation, themixture can be blended for a duration of time in the range of 10-15minutes. Other durations of time may be used depending on the overallamount or composition of the mixture.

In some implementations, the stability of the acid-in-scCO₂ emulsion canbe determined based, in part, on measuring the height of the emulsion.Over time, the acid in the emulsion can separate out of the emulsion,reducing the height of the emulsified portion relative to the height ofthe total emulsion mixture. The rate at which the acid separates out ofthe emulsion is indicative of the stability of the emulsion. A slowerrate of acid separation can indicate a more stable emulsion, and afaster rate of acid separation can indicate a less stable emulsion. Theheight of the emulsified portion can be measured in response to mixingthe nanoparticles, the supercritical CO₂, and the acid. Measuring theheight of the emulsified portion in response to mixing can be, forexample, measuring the height after mixing or measuring the height whilemixing. The height of the emulsified portion can be measuredperiodically and compared with the height of the total emulsion mixturethat includes the height of the separated acid. In this manner, the rateof acid separation and thus the stability of the emulsion can bedetermined. If the emulsion height measurement shows that the emulsionis insufficiently stable, the relative amounts of the nanoparticles, thesupercritical CO₂, and the acid can be adjusted as necessary to improvestability.

FIG. 3 is a schematic diagram of an example system 300 to manufacture anacid-in-scCO₂ emulsion. The system 300 can, for example, implement someor all of process 200. System 300 includes a cell 302 that is connectedto a CO₂ supply 308 and an acid supply 314. In some implementations, thecell 302 is connected to a nanoparticle supply 312, as shown in FIG. 3.The system 300 can include valves, piping, tubing, seals, fasteners, orother components that facilitate operation.

The cell 302 can be a tank, chamber, container, or other enclosed orsealed volume that can withstand temperatures and pressures sufficientfor containing supercritical CO₂. For example, cell 302 can be a hollowmetal cylinder. The cell 302 can be made of a metal such as aluminum orsteel or other metal, or be made of another material. In someimplementations, the cell 302 includes a blending device such as ahigh-shear mixer to mix the contents of the cell 302. In someimplementations, the cell 302 includes a window 306. Window 306 is atransparent window that allows the interior of the cell 302 to be seen.For example, the window 306 can allow the emulsion to be observed, andthe emulsion height to be seen and measured. The window 306 is able towithstand temperatures and pressures associated with supercritical CO₂and can be made of glass, plastic, or another material.

The cell 302 can also include a heat source 304. The heat source 304 canbe integrated into the cell 302 or be a separate component that iscoupled to the cell 302. The heat source 304 provides the heat thatheats or maintains the temperature of the CO₂ in the cell 302. The heatsource 304 can be a resistive heat source, a radiative heat source, orsome other type of heat source.

The CO₂ supply 308 supplies the CO₂ used in the emulsion. The CO₂ supplycan be a tank, vessel, chamber, Dewar, or other volume. The CO₂ supply308 can be integrated into the cell 302 (e.g., as an additional chamber)or be a separate component that is connected to the cell 302 (e.g., bypiping). The CO₂ supply 308 can contain CO₂ in a solid, liquid, gaseous,or supercritical state. The CO₂ supply 308 can contain CO₂ in apressurized state. In some implementations, the CO₂ supply supplies CO₂to the cell 302 through a pressurizer 310. The pressurizer 310 canpressurize the CO₂ from the CO₂ supply 308 before the CO₂ is transferredto the cell 302. For example, the pressurizer 310 can receive gaseousCO₂ from the CO₂ supply and pressurize the gaseous CO₂ to convert it toliquid CO₂. The pressurizer 310 can be an accumulator, pump, or othertype of pressurizing system.

The acid supply 314 can be a tank, vessel, chamber, or other volume thatcan supply acid to the cell 302. In some implementations, the acidsupply 314 holds a specific quantity (i.e., a premeasured amount) ofacid. The acid supply 314 can be integrated into the cell 302 (e.g., asan additional chamber) or be a separate component that is connected tothe cell 302 (e.g., by piping). In some implementations, the acid supply314 supplies acid to the cell 302 at a measured rate, such as adrop-wise rate.

The cell 302 can be connected to a nanoparticle supply 312 that suppliesnanoparticles to the cell 302. The nanoparticle supply 312 can be acontainer, vessel, chamber, port, or other component that can supplynanoparticles to the cell 302. In some implementations, the nanoparticlesupply 312 holds a specific quantity (i.e., a premeasured amount) ofnanoparticles. The nanoparticle supply 312 can be integrated into thecell 302 (e.g., as an additional chamber) or be a separate componentthat is connected to the cell 302 (e.g., by piping). In someimplementations, the nanoparticles are added to the cell 302 before thecell 302 is sealed, heated, or pressurized. In some implementations, thenanoparticles are introduced into the cell 302 through an airlock orother transfer chamber. In this manner, the nanoparticles can betransferred to the cell even if the cell is heated or pressurized.

FIG. 4 is a diagram illustrating an example well system 400. The examplewell system 400 can implement some or all of process 200 to manufacturean acid-in-scCO₂ emulsion. The well system 400 can flow acid-in-scCO₂emulsion 118 into a subterranean formation 406, as described below. Theexample well system 400 includes a wellbore 410 below the terraneansurface 402. The example wellbore 410 is cased by a casing 412. Awellbore 410 can include any combination of horizontal, vertical,curved, and/or slanted sections.

The well system 400 includes a working string 416 that resides in thewellbore 410. The working string 416 terminates above the surface 402.The working string 416 can include a tubular conduit of jointed and/orcoiled tubing configured to transfer materials into and/or out of thewellbore 410. The working string 416 can be in fluid communication withan emulsion supply 420 that supplies the acid-in-scCO₂ emulsion 418. Theemulsion supply 420 supplies acid-in-scCO₂ emulsion 418 to the workingstring 416 via a transfer system 422 of conduits, pumps, piping, andother related equipment. The working string 416 can communicate a fluidsuch as the acid-in-scCO₂ emulsion 418 into or through a portion of thewellbore 410.

The casing 412 can include perforations 414 in a subterranean region orzone, and the acid-in-scCO₂ emulsion 418 can flow into a formation 406through the perforations 414. The acid-in-scCO₂ emulsion 418 can be usedto stimulate formation 406, as described previously. In instances wheresome or all of the wellbore 410 is left open in an “open holeconfiguration” coinciding with the formation 406, the acid-in-scCO₂emulsion 418 can flow through the open hole wall of the wellbore 410.Additionally, resources (e.g., oil, gas, and/or others) and othermaterials (e.g., sand, water, and/or others) may be extracted from theformation 406. The casing 412 or the working string 416 can include anumber of other systems and tools not illustrated in the figures.

In some instances, some or all of the example process 200 can be used toproduce acid-in-scCO₂ emulsion 418 for use in the well system 400. Theacid-in-scCO₂ emulsion 418 can be produced at the well system 400 siteor produced off-site and transported to the well system 400 site. Forexample, some or all of process 200 can be implemented by emulsionsupply 420 to produce the acid-in-scCO₂ emulsion 418. In someimplementations, the acid-in-scCO₂ emulsion 418 can be produced in asystem like system 300 shown in FIG. 3. In some instances, thesupercritical CO₂ in the acid-in-scCO₂ emulsion 418 remains in asupercritical state once transported downhole. In some instances, CO₂can be supplied downhole and the CO₂ be converted to supercritical CO₂downhole. In this manner, the acid-in-scCO₂ emulsion 418 can bemanufactured downhole.

In some implementations, the acid-in-scCO₂ emulsion can be formulated tocontrol the stimulation of the formation. Based on the characteristicsof the formation (e.g., size, porosity, composition, etc.), thecomposition of the acid-in-scCO₂ emulsion can be specified to obtain anidentified acid retardation rate or rock reaction rate within theformation. For example, the quantity of CO₂ in the stimulant (e.g., theproportion of scCO₂ in an acid-in-scCO₂ emulsion) can be determined toobtain a specific acid retardation rate or rock reaction rate for anidentified formation. For example, an acid-in-scCO₂ emulsion with alower proportion of CO₂ has more acid available to react with theformation, and thus can have a lower acid retardation rate than anacid-in-scCO₂ emulsion with a higher proportion of CO₂. In someimplementations, the CO₂ flowed into the formation is in a gaseous stateor liquid state. CO₂ in a liquid or supercritical state has greaterviscosity than CO₂ in a gaseous state, thus can increase the retardationrate of the acid in the formation. In some cases, the acid-in-scCO₂emulsion has a rock reaction rate approximately 25% to 50% that of HCl.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of the disclosure.

What is claimed is: 1-20. (canceled)
 21. An emulsion comprising: aninternal phase comprising acid; an external phase comprisingsupercritical carbon dioxide; and nanoparticles that stabilize theinternal phase and the external phase.
 22. The emulsion of claim 21,wherein the acid comprises hydrochloric acid, citric acid, formic acid,acetic acid, hydrofluoric acid, or a chelating agent, or anycombinations thereof
 23. The emulsion of claim 22, wherein a ratio of aconcentration of the acid to a concentration of the supercritical carbondioxide is in a range between 30% and 70%.
 24. The emulsion of claim 21,wherein the acid comprises hydrochloric acid.
 25. The emulsion of claim21, wherein the nanoparticles comprise hydrophobic nanoparticles, andwherein a concentration of nanoparticles in the emulsion comprises atleast 0.1% by weight.
 26. The emulsion of claim 21, further comprising acorrosion inhibitor.
 27. The emulsion of claim 26, wherein aconcentration of the corrosion inhibitor in the emulsion is in a rangeof between 0.25% and 0.6% by volume.
 28. A method comprising: forming anemulsion for stimulation of a formation, wherein forming the emulsioncomprises: mixing nanoparticles and carbon dioxide at a temperature anda pressure sufficient to convert the carbon dioxide into supercriticalcarbon dioxide; and mixing acid with the nanoparticles and thesupercritical carbon dioxide to give a mixture, wherein the acidcomprises hydrochloric acid, citric acid, formic acid, acetic acid,hydrofluoric acid, or a chelating agent, or any combinations thereof.29. The method of claim 28, comprising blending the mixture to give theemulsion.
 30. The method of claim 28, wherein the temperature is atleast 40° C. and the pressure is at least 1100 pounds per square inch(psi).
 31. The method of claim 28, comprising applying heat to give thetemperature sufficient to convert the carbon dioxide into supercriticalcarbon dioxide.
 32. The method of claim 28, wherein mixing the acid withthe nanoparticles and supercritical carbon dioxide comprises adding theacid at a drop-wise rate.
 33. The method of claim 28, comprising addinga corrosion inhibitor to the acid prior to mixing the acid with thenanoparticles and supercritical carbon dioxide.
 34. The method of claim33, wherein concentration by volume of the corrosion inhibitor in themixture is at least 0.25%.
 35. A method of manufacturing an emulsion,comprising: mixing carbon dioxide with nanoparticles; applying heat toconvert the carbon dioxide to supercritical carbon dioxide; and mixingacid with the supercritical carbon dioxide and nanoparticles at a ratioof the acid to the supercritical carbon dioxide in a range 30% to 70%.36. The method of claim 35, wherein the acid comprises hydrochloricacid.
 37. The method of claim 35, comprising pressurizing the carbondioxide prior to mixing the carbon dioxide with the nanoparticles. 38.The method of claim 35, comprising pressurizing the carbon dioxide to atleast 1500 pounds per square inch (psi).
 39. The method of claim 35,wherein applying heat comprises increasing a temperature of the mixtureof carbon dioxide and nanoparticles to at least 40° C.
 40. The method ofclaim 35, wherein the nanoparticles comprise hydrophobic nanoparticles.